I'm new to this, and I'm trying to figure out what hardware I would need for this project. I want to be able to open my gate with my iphone. There is a low voltage wire coming from the gate to the house where there is a pushbutton that shorts the wires when you hold it. I thought if I spliced in another set of wires and hooked those wires to an arduino, I could send it a signal from my iphone through wifi that would trigger the arduino to short the wires for one second and then release. What would be the cheapest and easiest way to do this?

You may be able to 'short the wires' by switching on a transistor, but it depends on the voltage and current of the gate. To open it with wiFi would probably be hard, but you could use the ethernet shield to create a simple web page that allows you to open the gate, or use the GSM shield to send a text to your gate.

Yes, the arduino would have the shield, then when a client connects (or connects and enters a password) the gate can be made to open. A relay would be a better idea than the transistor, as long as you wire it up correctly.

It took me a little time to get my hands on a multimeter, but I tested the wires, and they fluctuate between 2 and 3 volts dc. My brother said it fluctuates because the gate opener has a battery backup. Can you suggest what hardware I should buy? Would this work for me instead of buying an arduino and an ethernet shield separately? http://arduino.cc/en/Main/ArduinoBoardEthernet

It looks like the 'Ethernet' board would be perfect, although you'll need a connector to upload your program.

For the hardware, I'd use a relay. Wire it up as shown here: http://arduino.cc/playground/uploads/Learning/solenoid_driver.pdf, only with a relay in place of the solenoid. Connect the relay contacts up to the wires for the gate, then when the arduino receives something from a web client saying to turn it on, the gate will (should) open.